constant (ˈkɒnstənt)  
—adj  
1.  fixed and invariable; unchanging 
2.  continual or continuous; incessant: constant interruptions 
3.  resolute in mind, purpose, or affection; loyal 
—n  
4.  something that is permanent or unchanging 
5.  a specific quantity that is always invariable: the velocity of light is a constant 
6.  a. maths a symbol representing an unspecified number that remains invariable throughout a particular series of operations 
b. physics a theoretical or experimental quantity or property that is considered invariable throughout a particular series of calculations or experiments  
7.  See logical constant 
[C14: from Old French, from Latin constāns standing firm, from constāre to be steadfast, from stāre to stand]  
'constantly  
—adv 
constant con·stant (kŏn'stənt)
adj.
Continually occurring; persistent.
Unchanging in nature, value, or extent; invariable.
A quantity assumed to have a fixed value in a specified mathematical context.
An experimental or theoretical condition, factor, or quantity that does not vary or that is regarded as invariant in specified circumstances.
constant (kŏn'stənt) Pronunciation Key

A number that appears in equations and formulas and does not vary or change. Examples are Planck's constant and the speed of light.